Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 240128 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 928 PM EDT Sun Jul 23 2017 .SYNOPSIS... High pressure remains off the southeast coast. A weak boundary will remain near the area through Monday before stalling south and east of the area Tuesday. An upper level trough of low pressure will move over the northeastern US early in the week. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... Have had a couple of waves of showers/thunderstorms so far this afternoon/early evening...the most prolific of which dropped between 3-5 inches (radar estimated) across northeast Maryland. The forecast area presently in a lull radar-wise, but that likely won`t be the end of the precip potential tonight. Mesoscale analysis suggests 1000-1500 J/kg MLCAPE still present across much of Maryland and adjacent Virginia. The 00Z LWX RAOB confirms this. Effective shear, fortunately, a bit less than this afternoon, but still around 20-25 kt. A cap evident around 500 mb on the sounding. As a weak surface boundary drops toward the forecast area tonight, subtle height falls likely will erode the inhibition, making it possible for another round of convection...unrelated to the activity currently in the upper Ohio Valley. RAP/HRRR composite suggests that showers/thunderstorms will develop somewhere near the Blue Ridge/Catoctins just after dark and track toward Baltimore and Washington toward midnight. After that, details diverge. But given recent rains (and ongoing flooding) and precipitable waters at or above 2 inches in the metros...will be keeping the Flash Flood Watch in place through the early overnight. Would still not rule out localized severe weather. However, if will take a more vigorous updraft to collapse and produce downburst winds. Some CAMs suggesting thats possible...even after midnight. Otherwise, there will a decreasing trend between midnight and sunrise. Will be utilizing the latest obs and composite meso models to fine tune temperature forecasts. && .SHORT TERM /6 AM MONDAY MORNING THROUGH TUESDAY/... Frontal boundary will finally try to push through the area Monday. Most guidance continues to be stubborn with convective coverage...though believe at least isolated showers and storms are possible as the front encounters a warm and unstable airmass. The strongest storms would be capable of gusty winds and locally heavy rainfall. Some improvement from the heat front stalls to the S/E of the area. High temperatures will generally top out in the mid to upper 80s. Dewpoints will fall back into the 60s...which should feel more pleasant after the extended stretch of hot and humid weather. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Wednesday should be the pick day of the week with high pressure centered over New England, and extending down over our region. That should keep temperatures slightly below normal, and humidity in reasonable check with dewpoints in the 60s. By the time we get to Thu, the high has slipped off to the east and southwest flow returns with a rebound in temps and humidity. Late in the week, probably Friday, our next cold front is slated to push through from the northwest. Current indications are that it will stall just offshore with an area of low pressure forming along it off the Delmarva Sat and Sun. && .AVIATION /02Z MONDAY THROUGH FRIDAY/... VFR conditions will prevail through Monday...outside of thunderstorm activity. Another wave of showers and thunderstorms will be possible late this evening into the overnight hours. Flight restrictions will be brief, vsby more than ceiling, related to the rainfall. There may be some gusty winds, but confidence in details low. An isolated storm is also possible Monday...though most sites should remain dry. VFR returns Tuesday. VFR conditions expected Wed and Thu. Lowered vsbys and cigs are possible on Friday from the expected cold front with its showers and thunderstorms. && .MARINE... Mesoscale models have consistently been suggesting that 20 kt wind gusts possible up the Bay in southerly channeling tonight. Based on this consistency and presence of outflows from nearby showers/storms, have opted to raise a Small Craft Advisory for the mid Bay and mid-lower tidal Potomac through 4am. (Threat of the Potomac likely south of the Route 301 Bridge.) In addition, thunderstorms capable of producing gusty winds above Special Marine Warning criteria will be possible tonight. A couple of thunderstorms will be possible on Monday as well. Otherwise, winds will be below SCA thresholds. No marine hazards Monday night or Tuesday. Other than thunderstorms (most likely on Friday), no widespread marine threats are anticipated Wed through Friday. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...Flash Flood Watch until 3 AM EDT Monday for DCZ001. MD...Flash Flood Watch until 3 AM EDT Monday for MDZ004>006-011-013- 014-503>508. VA...None. WV...None. MARINE...Small Craft Advisory until 4 AM EDT Monday for ANZ533-534-536- 537-543. && $$ SYNOPSIS...MSE NEAR TERM...HTS SHORT TERM...MSE LONG TERM...MSE/CAS AVIATION...HTS/MSE/CAS MARINE...HTS/MSE/CAS is the U.S. government's official web portal to all federal, state and local government web resources and services.